1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery for high output use, and more particularly, to an inexpensive and safe non-aqueous electrolyte secondary battery and a battery pack using the battery.
2. Background Art
Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have a higher energy density than other storage batteries. Because of this advantage, non-aqueous electrolyte secondary batteries are expanding their market from consumer use such as portable appliances to power tool use such as electric tools.
In hybrid electric vehicle use, for example, if only a large amount of current can be quickly taken out of a non-aqueous electrolyte secondary battery when the vehicle is started or accelerated, the vehicle can be driven afterwards by the internal combustion engine. On the other hand, in electric tool use where the non-aqueous electrolyte secondary battery is the only power source, a large load is required to start the motor. Particularly when the motor is started from the condition in which the tool is in contact with the operating object, the battery is required to produce a larger output.
These operating circumstances require high-output secondary batteries to improve their output characteristics, for which a reduction in internal resistance is inevitable. The reduction in internal resistance is greatly affected by the current collecting structure in the electrodes (the positive electrode and the negative electrode). More specifically, the internal resistance can be reduced by attaching a current collector collectively to the wound parts of an exposed portion having no mixture applied thereon of each core sheet of the electrodes. This technique is already in practical use in nickel-cadmium batteries and nickel-hydrogen batteries in electric tool and hybrid electric vehicle uses. In these batteries, the core sheets are at least as large as 300 μm in thickness, making it possible to attach a current corrector collectively to the wound parts of an exposed portion at an end of each electrode.
In non-aqueous electrolyte secondary batteries, on the other hand, the core sheets only have a thickness of several tens of micrometers. Therefore, the core sheets need to have a core-exposed portion at their ends so that a current collector can be attached collectively to the wound parts of the core-exposed portion. As methods for forming the core-exposed portion, there have been various suggestions. For example, Japanese Patent Unexamined Publication No. H10-144301 suggests removing part of the mixture layers formed on each electrode. Japanese Patent Unexamined Publication No. H11-354110 suggests protecting an area that is to be the core-exposed portion with a tape and then removing the tape after the area is coated with mixture layers.
In these methods, however, the process of removing the mixture layers degrades the productivity, and the use of the masking material, which is an expendable supply, not only requires the applying and removing processes, but also boosts the cost.
Japanese Patent Unexamined Publication No. 2003-20890, on the other hand, suggests forming the mixture layers excluding the end portions of the electrodes so as to make the core-exposed portions at the end portions. In this case, the mixture is applied in such a manner as to swell on the boundary between the core-exposed portion and the mixture layer, and the thickness of the mixture layer is smoothed in a later rolling process.
The formation of the electrodes in this manner causes a problem according to safety. More specifically, the application of the mixture in such a manner as to swell on the boundary between the core-exposed portion and the mixture layer of the positive electrode increases the weight of the positive electrode on the boundary. This causes the negative electrode to have a load exceeding the load design value at the portion opposing the boundary on the positive electrode. As a result, lithium ions that cannot be stored in the negative electrode may deposit as metallic lithium on the surface of the negative electrode. It is known that an increase in the amount of lithium ions to be stored in the negative electrode leads to a decrease in the thermal stability of the negative electrode. That is why the load design of the negative electrode is very important. Especially, high-output secondary batteries are required to be large in size for the purpose of improving output characteristics. The increased output and size causes an increase in the internal energy and a decrease in the thermal stability of the non-aqueous electrolyte secondary batteries. Therefore, the electrode design is very important to the batteries.